>not good to have two (batteries) in parallel, if 1 shorts youve got a
tremendious drain
>on the other in an instant, check out the recreational veh. people they have
>battery isolators.
Let's look at the physics of the matter. How do batteries short?
(-) to (+) terminal or one-cell-at-a-time? If a 12-volt battery
shorts a single cell, it becomes a 10-volt battery. Now the ideal
charging voltage for the battery becomes 11.5 volts as opposed to
the 13.8 or so that exists on the bus.
How does the system react? The battery becomes another load on
the system. How much of a load? Consider the following: If ideal
charging voltage is 2.3 volts per cell (13.8/6) and we've now
got a 10 volt battery (13.8/5) the voltage applied to each cell
is now 2.76 volts . . . an increase of .46 volts per cell. Hmmmm,
how much current will a cell draw if "overcharged" by .46 volts?
We could find out by applying 16.5 volts to a fully charged battery.
When this test was conducted in the laboratory on the B&C RG
batteries, the charging current eventually fell to less than 1 amp
and the battery temperature rose less than 10 degrees C. The battery
quietly went south.
From experience, I'll guess that it won't exceed 20 amps and that
for only a short time. If it's a "flooded" battery, the liquid in
the working cells begins to disassociate rapidly into hydrogen/
oxygen gasses . . . plates get bubbles on them and internal resistance
tends to rise. RG batteries have so little water in them that severe
abuse tends to "poof" the littel bit of water out the pressure
relief valves and it's all over.
Flooded batteries will vent a lot of hydrogen/oxygen in this mode
and quite frankly, THIS is the real hazard to continued flight.
They may spit liquid water/acid out the caps too. This is why
they need a battery box. The RG battery will sit there and quietly
die . . . you won't know it happened until you check the battery
during next preflight.
Now, let's assume there's another battery in parallel with the first.
The alternator is probably going to carry the excess 20 amps or
so needed to supply the extra demand of the failing battery.
Bus voltage will stay up where it belongs
and except for an abnormally high alternator loadmeter reading,
you probably won't notice anything. If the added load causes
the alternator to go into current limit, the bus voltage sags.
How far? Depends on the total load and some alternator characteristics.
If below 13.0 volts you SHOULD get a low volts warning light to
tell you something is amiss. But in any case, the votage isn't
going to drop enough to even begin to discharge the battery
that's still okay. Recall that a battery can DELIVER energy
at 12.5 volts or LESS.
Once you get a low-voltage light, if you want to play airborne
systems analysis and/or mechanic, you could turn the batteries
off one at a time to see if the low-volts light goes out. When
you do de-select the failed battery, the bus will come back up
and light will go out. On the other hand, if you have developed
a "plan-b" for electrical systems malfunctions, flipping three
switches at most automatically selects the alternate mode of
operation and you get back with the piloting task at hand.
The important item to note from this analysis is that while one
battery is dying, the otherone is not affected in any way that
requires immediate attention from the pilot.
basicaly two high current diodes, alt power goes into the
>common and comes out as 2 isolated outputs 1 for each battery. worked great
>on my race boat and rv, now its my planes turn.
Battery isolators have been around for years for a variety
of applications none of which have anything to do with protecting
one battery from the ravages of another battery. They are
commonly used in situations where there is minimal instrumentation
and no need for "Plan-B" development and implimentation.
Their primary value is to allow one to charge a battery in
a trailer/boat from the towing vehicle system while preventing
the towed appliance from loading the vehicle battery. I had
an uncle who stuck himself way out in the boonies because
he'd run down BOTH batteries while sitting and could not
start his truck.
The reason we use paralleled batteries in airplanes is so
that they may share engine cranking duties by virtue
of high-current, hard connections to the system via
battery contactors. By making them the same size, we
can rotate the installed batteries once a year . . . put
a new one in the main slot, move the 1 year old main to
the aux slot and put the old aux battery in your lawn
mower. Next year, repeat the exercise.
When no battery is more than two years old and one battery
is always less than one year old, the likelyhood of the
shorted cell scenario even occuring is very, very
close to ZERO. If it should happen, it's no big deal.
In the mean time, you enjoy a VERY reliable power
generation/storage system for keeping essential goodies
running.
Bob . . .
AeroElectric Connection
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